Compositions which cure at room temperature to form a silicone elastomer have been known in the art, and such compositions have been widely used in the industry. The mechanisms of the room temperature curing include the one in which the curing is realized by hydrosilylation, the one realized by UV irradiation, the one realized by condensation between hydroxy group and a hydrolyzable group bonded to silicon atom, and the like. Among these, the polyorganosiloxane composition which cures by condensation is well adapted for applications such as building sealant since it is capable of curing and accomplishing the adhesion at room temperature, and it is less likely to experience curing inhibition by the impurities as often encountered in the case of the curing by hydrosilylation.
In the case of building sealants, the cured rubber is often required to exhibit low elasticity corresponding to the category of low to medium modulus in modulus at 50% elongation (M50) defined by Japan Sealant Industry Association. In order to meet such requirements, various techniques have been proposed, and exemplary such techniques include dilution with a silicone oil having no reactive group and addition of various plasticizers. While these techniques showed some effects in realizing the low modulus, prolonged use of such sealants often caused staining of the joint surface and the staining of porous materials such as natural rocks due to the bleeding, and improvement of such drawbacks has been awaited.
Prevention of the staining can be realized by using a non-reactive silicone oil or by using a composition which does not include the silicone oil or the plasticizer. Such composition, however, is associated with the problem of high modulus after the curing and insufficient adhesion to metals and various coatings. In order to produce a soft rubber which does not contain a plasticizer, attempts have been made to reduce crosslink density after the curing by using a combination of a difunctional crosslinking agent having chain extending ability with a polyfunctional crosslinking agent to thereby extend the polydiorganosiloxane chain during the curing simultaneously with the crosslinking. Typical such commercially available compositions include those called aminoxy type using a siloxane having a N,N-dialkylaminoxy group for the crosslinking agent, and those called amide type using a siloxane having N-alkyl acetamide group for the crosslinking agent. These compositions, however, have been associated with the problems of insufficient storage stability of the composition and curing inhibition caused by the presence of a compound having active hydrogen such as an alcohol.
Production of a rubber having a low crosslink density and good storage stability which does not experience curing inhibition is proposed in Patent Document 1 (JP-A 63-83167) which involves use of a functional group which has been widely used in the art. While this method uses RNHCH2MeSi(OMe)2 having chain extending ability, economical production of this compound has been extremely difficult, and stable use of such compound in good balance with other crosslinking agent has also been difficult. Also proposed are methods using a polydiorganosiloxane having hydroxy group at one end and a trialkylsiloxy group on the other end (Patent Documents 2 and 3: JP-A 9-151326 and JP-A 2004-182942). When the polydiorganosiloxane having hydroxy group at one end is produced by equilibrated polymerization of an organocyclopolysiloxane using an alkali or acid catalyst, namely, by a method commonly used in the art, non-reactive polydiorganosiloxane is produced as a by-product. Since removal of this non-reactive by-product polydiorganosiloxane has been impossible, use of this composition has been associated with the problem of staining, for example, staining of the joints of the natural rocks by bleeding. Production of such non-reactive polydiorganosiloxane can be reduced by using a method in which ring-opening polymerization of cyclic siloxane trimer is conducted by using trimethylsiloxy lithium for the initiator; or by using a method in which the polymerization is conducted in the presence of a pentacoordinated silicon compound using trimethyl hydroxy silane for the initiator. These methods, however, are economically disadvantageous since both the monomer and the catalyst are expensive.
As described above, there has so far been no sealant which is less likely to stain the surface such as the surface of a natural rock by bleeding, and which cures into a soft rubber having a low to medium modulus favorable for a sealant as well as excellent adhesion and improved storage stability.